For example, in digital communications, it is known that the phase noise of a Voltage Controlled Oscillator (VCO) is greater at low frequencies with respect to a carrier frequency due to an increase of flicker noise power magnitude at low frequencies. Injection locking allows phase tracking of a reference clock. It is for this reason that VCOs are generally employed locked to a clean reference clock source to filter out low-frequency intrinsic phase noise of the VCO. Due to positive feedback that enables VCOs to oscillate, the noise created by every element in a VCO within a phase locked loop gets amplified over time. This positive feedback noise enhancement is referred to as jitter accumulation and is schematically illustrated in FIG. 1. For the purpose of concise description herein, jitter accumulation can be summarized as a timing uncertainty in VCO output signal transitions, transition timing uncertainty which grows over time.
In particular, injection locked VCOs exhibit jitter accumulation dependent on the effective bandwidth achieved by the injection locking process. Employing a higher reference clock frequency would result in a higher effective bandwidth and better jitter performance. However, a main shortcoming of employing traditional injection locking is the fact that jitter accumulation is directly proportional to the reference clock frequency.
There is a need to reduce jitter accumulation in VCOs for a variety of applications.